Researchers measured the length of time they cried over successive nights, as their mothers waited in a nearby room.

By the third night of the study, infants were found to have cried for a shorter period of time before falling asleep.

But the study, published in the journal Early Human Development, found the levels of cortisol in their saliva remained high.

In mothers, who could hear their babies, the level of the hormone fell in accordance with the time spent crying, indicating they had relaxed as the youngsters appeared to settle.

The study found: "On the third day of the program, results showed that infants' physiological and behavioral responses were dissociated. They no longer expressed behavioral distress during the sleep transition but their cortisol levels were elevated."

Wendy Middlemiss told the Daily Mail: “Although the infants exhibited no behavioural cue that they were experiencing distress at the transition to sleep, they continued to experience high levels of physiological distress, as reflected in their cortisol scores.